Edelaar P. & Terpstra K. (2004) The nominate subspecies of the Common Crossbill Loxia c. curvirostra polytypic? I. Morphological differences among years at a single site.
ARDEA 92 (1): 93-102

The discovery of discrete vocal types of Common Crossbill in Western Europe opens the possibility that the nominate subspecies Loxia c. curvirostra in fact consists of a group of cryptic, vocally differentiated and reproductively isolated sibling species, reflecting a similar situation in North America. We compared measures of Common Crossbills collected at a single Dutch site by a single observer from 1983 to 2001. During 1983-84 - and to a lesser extent also in 1985-88 and 1992 - Common Crossbills had relatively long wings, low body masses, and deep but short bills compared to other years. Changes in methods or phenotypic flexibility of the measures do not explain these results. Biometric differences among years are likely linked to the proportional abundance of different populations of Common Crossbills at the catching site. A difference between years in the relationship between wing length and bill depth supports the idea that differences between populations are the result of selection, not neutral differentiation. These results are consistent with the variable presence of multiple, at times sympatrically occurring, cryptic species